As a method for locking and unlocking doors of a vehicle, a smart entry method, an advanced style of a keyless entry method, has become used in recent years. In a keyless entry method, doors of a vehicle are locked or unlocked when a user presses a button provided on a key. The user is required to take up the key from a bag or a pocket to use the key.
On the other hand, a smart entry method has a function for performing near field communications between a vehicle and a key (hereinafter referred to as “smart key”) to lock or unlock doors of the vehicle through radio communications between the vehicle and the smart key. Specifically, with the smart key being held in a bag or a pocket, a user can unlock the doors of the vehicle, when the doors are locked, by only touching a touch sensor or a request switch of the vehicle, as well as can lock the doors of the vehicle, when the doors are unlocked, by only touching the touch sensor. In addition, in the smart entry method, when the smart key is detected in the vehicle, an engine can be started without inserting the key into a keyhole of the vehicle. Accordingly, it is necessary that the smart key be precisely detected whether the smart key is present inside or outside the vehicle.
The smart entry method uses radio waves, thus, if the smart key is shielded by a metallic object in a cabin, for example, the smart key could unintentionally be locked in the cabin.
As a technology for preventing a smart key from being locked in a vehicle, a technology disclosed in Patent Literature 1 is known, for example. Patent Literature 1 discloses a vehicular controller that prevents a vehicle from being remotely controlled when the vehicular controller is switched from a detection mode in which positional information of a portable device can be detected in a cabin to a non-detection mode in which positional information of the portable device cannot be detected. According to this vehicular controller, even if a radio wave between the vehicle and the portable device is interrupted due to a dead battery of the portable device while the portable device is present in the cabin, a user can prevent the portable device from being locked in the cabin.